Electrical current rectifier



United States Patent assignors to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Application February 4, 1955 Serial No. 486,285

4 Claims. (Cl. 117-400) This invention relates to rectifiers for electrical current. More specifically, this invention relates to semiconductor rectifiers, as distinguished from vacuum tube rectifiers.

In the field of electronics there is an increasingly great demand for small, rugged, shock-resistant rectifiers which can be operated Without filament circuits (thereby eliminating need for filament power supply, warm up period, etc.). At the present time there are essentially only three types of rectifiers which meet these requirements, namely, the copper-copper oxide rectifiers, the selenium rectifiers and the so-called crystal rectifiers (germanium and silicon). The first two of these have the very serious disadvantage of a poor frequency response, giving very little rectification of alternating current above audio frequencies. All except the siilcon crystal rectifiers are somewhat limited by poor temperature characteristics, being unsuitable for operation at temperature above 85 to 100 C. The crystal rectifiers are decidedly better than the others with respect to frequency response, but they are very expensive, and consequently cannot be used for as many applications as would otherwise be desirable.

Consequently, it is an object of the present invention to provide a relatively inexpensive semiconductor rectifier having a good frequency response and high tempera- ,ture stability.

It has now been found that the boundary or interface between titanium sulfides and titanium oxides has the surprising property of presenting a very high resistance to electrical current passing across the boundary in one direction, but a very low resistance to electrical current passing across the boundary in the reverse direction. More specifically, it has been found that a compact mass made up of discrete portions of titanium sulfide and titanium oxide will function as an electrical current rectifier. It has also been found that such rectifiers can be readily and conveniently formed by the oxidation of a limited portion of a mass of titanium sulfide while leaving the remainder of the titanium sulfide unreacted.

By way of illustration, a pellet of titanium disulfide compressed to 80 percent of theoretical density was placed upon a cooled aluminum plate. The exposed face of the pellet was then heated in open air with a natural gas flame to coat the surface with a white titanium oxide layer about 0.03 inch thick. Resistance and frequency response of the oxide-coated sulfide pellet were then measured. With 1.5 volts impressed across the pellet, the resistance was found to be from 50 to 200 times greater (depending upon the particular surface areas contacted) in one direction than in the reverse direction-the higher resistance being obtained when the oxide portion of the pellet was positive and the sulfide portion was negative. Frequency response was measured by placing the pellet in series with a beat frequency oscillator and D. C. ammeter, impressing an alternating current of constant voltage across the pellet and varying the frequency. It was found that the net unidirectional current flow through the ammeter varied only from microamperes at 1000 cycles per second to 3 microampercs at 1,000,000 cycles per second.

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rn an be ap earing; areutasin -ainfiueuauium oxide junction canbeobtained-"'by nietliods other than the above-described oxidation ofthe sulfideone obvious variation being .the ,sulfurizatioh' of the oxide. Thisfl'tter inethod is "-sofiiewuamess attractive than the -fir'st ho-wever, since it is considerably more d'ifli'ciilt -to commune final thicknes of the oxide portion of the pellet when sulfurizing a mass of the oxide. (The electrical resistance of titanium oxide is quite high, whereas the resistance of titanium sulfide is very low-the conductivity of pure crystals of pure titanium disulfide approaching that of mercury; thus, it is very desirable that the oxide portion of the sulfide-oxide rectifiers be as thin as possible in order to reduce the internal resistance of the rectifier.)

Theoretically, it might also be possible to obtain the present rectifiers by preparing the titanium sulfide and the titanium oxide in physically separate portions and thereafter placing them together. However, as a practical matter, it would be an extremely difficult task to achieve sufficiently intimate contact between the oxide and the sulfide to produce an effective rectifying boundary between the two. Thus, a preferred embodiment of the present invention is the formation of one of the semiconductor materials from a portion of the other semiconductor material, in a manner such as described above.

The preferred sulfide for use according to the present invention is titanium disulfide, but other titanium sulfides, such as titanium monosulfide, titanium sesquisulfide, titanium trisulfide, or mixtures of two or more of these various sulfides can also be used. Likewise, titanium dioxide is the preferred oxide, but any of the other nu-.

merous oxides, or mixtures thereof, can be used.

As stated earlier, the preferred method of forming the present semiconductor rectifiers of the present invention involves the surface oxidation of a mass of titanium sulfide. The term oxidation is used herein in the restricted sense of formation of an oxiderather than in the broad sense of increasing the energy state of the valence electrons in the titanium atom. The oxidation of the sulfide according to the present invention can be carried out by exposing the sulfide to oxygen or to an oxygen-containing atmosphere at elevated temperatures (as illustrated in the foregoing example) or with oxygen-containing oxidizing agents, such as hydrogen peroxide, nitric acid, etc. The reverse order of formation (i. e., sulfurization of the oxide) can be carried out by reaction of the oxide with sulfur, hydrogen sulfide, carbon disulfide or other similarly well-known sulfurizing agents. In all cases, precaution must be taken to insure that oxidation or sulfurization of only a portion of the sulfide or oxide, respectively, takes place while leaving the remaining portion thereof unreacted. A particularly simple and effective way to react a portion of the mass while retaining the rest unreacted is by chilling or cooling one portion of the mass so as to keep the temperature of that portion at a point at which the reaction will not take place. This chilling or cooling can be done as described earlier by placing the pellet or mass upon a sheet of cold metal or similar heat conductive material.

We claim:

1. An electrical current rectifier comprising a compact mass of discrete portions of an oxide and a sulfide of titanium in intimate contact with each other.

2. An electrical current rectifier comprising a compact mass of discrete portions of titanium dioxide and titanium disulfide in intimate contact with each other.

3. A method of making an electrical current rectifier, which comprises oxidizing a discrete portion of a compact mass of a sulfide of titanium without separating said portion from the remainder of said sulfide, thereby forming a compact mass made up of discrete portions of a 3 4 titanium sulfide and a titanium oxide in intimate contact References Cited in the file of this patent with each other. i

4. A method of making an electrical current rectifier, UNITED STATES PATENTS which comprises oxidizing a discrete portion of a corn- 929,582 Garrctson July 27, 1909 pact mass of a disulfide of titanium without separating 5 1,895,684 Ruben Jan. 31, 1933 said portion from the remainder of said sulfide, thereby 1,895,685 Ruben I an. 31, 1933 forming a compact mass made up of discrete portions of 1,949,383 Weber Feb. 27, 1934 titanium disulfide and titanium dioxide in intimate con- 2,711,496 Ruben Iune'21, 1955 tact with each other. 

1. AN ELECTRICAL CURRENT RECTIFIER COMPRISING A COMPACT MASS OF DISCRETE PORTIONS OF AN OXIDE AND A SULFIDE OF TITANIUM IN INTIMATE CONTACT WITH EACH OTHER. 